An efficient recyclable peroxometalate-based polymer-immobilised ionic liquid phase (PIILP) catalyst for hydrogen peroxide-mediated oxidation

Article abstract of DOI:10.1039/c2gc16679h

A linear cation-decorated polymeric support with tuneable surface properties and microstructure has been prepared by ring-opening metathesis polymerisation (ROMP) of a pyrrolidinium-functionalised norbornene-based monomer with cyclooctene. The derived peroxophosphotungstate-based polymer-immobilised ionic liquid phase (PIILP) catalyst is an efficient and recyclable system for the epoxidation of allylic alcohols and alkenes, with only a minor reduction in performance on successive cycles.

Full text of DOI:10.1039/c2gc16679h

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Green Chemistry
Cite this: Green Chem., 2012, 14, 925
www.rsc.org/greenchem
COMMUNICATION
An efficient recyclable peroxometalate-based polymer-immobilised ionic
liquid phase (PIILP) catalyst for hydrogen peroxide-mediated oxidation†
Simon Doherty,*a Julian G. Knight,*a Jack R. Ellison,a David Weekes,a Ross W. Harrington,a
Christopher Hardacre*b and Haresh Manyarb
Received 28th December 2011, Accepted 16th February 2012
DOI: 10.1039/c2gc16679h
in some cases the activity and selectivity matched that obtained
under homogenous conditions.
A linear cation-decorated polymeric support with tuneable
surface properties and microstructure has been prepared by
ring-opening metathesis polymerisation (ROMP) of a pyrro-
lidinium-functionalised norbornene-based monomer with
cyclooctene. The derived peroxophosphotungstate-based
polymer-immobilised ionic liquid phase (PIILP) catalyst is
an efficient and recyclable system for the epoxidation of
allylic alcohols and alkenes, with only a minor reduction in
performance on successive cycles.
As part of a recently initiated programme to explore and
develop the concept of polymer-immobilised ionic liquid phase
(PIILP) catalysis we used ring-opening metathesis polymeris-
ation (ROMP) to prepare new cation-decorated polymeric sup-
ports, reasoning that the well-behaved living nature of this
process and the functional group tolerance of the catalyst would
enable surface properties, microstructure, ionic microenviron-
ment, stability and porosity to be modified in a rational and sys-
tematic manner, which would enable catalyst–surface
interactions, substrate accessibility and efficiency to be opti-
mised, property–function relationships to be elucidated and new
activity–selectivity relationships to be established. Herein, we
report that peroxometalates can be immobilised on a pyrrolidi-
nium-decorated norbornene-based polymer and this material is
an efficient catalyst for the oxidation of allylic alcohols and
alkenes. The catalyst can also be recovered and reused with only
a minor reduction in performance.
Introduction
Polyoxometalates (POM) are an architecturally and structurally
diverse1 and fascinating class of redox active2 anionic transition
metal–oxygen-based materials. They have been shown to be
efficient Brønsted acid catalysts and highly selective oxidation
catalysts for a host of commercially important transformations.3
Surface immobilisation of polyoxometalates and their derived
peroxometalates has been intensely investigated as a potentially
practical approach to developing a continuous flow process,
improving robustness and recyclability as well as overcoming
the problem of catalyst leaching normally associated with liquid–
liquid biphasic systems. A number of strategies have been devel-
oped for the heterogenisation of these catalysts including electro-
static anchoring to ionic liquid-modified silica,4 nanosized ionic
dendritic polyammonium polycations,5 tripodal organic polyam-
moniums,6 alkylated polyethyleneimine,7 cationic hydrophobic
silica xerogels8 and a poly(ethylene oxide–pyridinium) matrix;9
Result and discussion
The pyrrolidinium-based monomer 4 was prepared according to
Scheme 1. The BCl3-catalysed Diels–Alder cycloaddition
between methacrylonitrile and cyclopentadiene was conducted in
aSchool of Chemistry, Bedson Building, Newcastle University, Newcastle
upon Tyne, NE1 7RU, England. E-mail: simon.doherty@ncl.ac.uk; Fax:
+44 0191 222 6929; Tel: +44 0191 222 6537
bSchool of Chemistry and Chemical Engineering, David Keir Building,
Queen’s University, Stranmillis Road, Belfast, Northern Ireland. E-mail:
c.hardacre@qub.ac.uk; Fax: +44 02890976524; Tel: +44 02890976524
†Electronic supplementary information (ESI) available: Synthesis and
characterisation of compounds 1–4, polymers 5 and 6, details of cataly-
sis and for compound 4 details of crystal data, structure solution and
refinement, atomic coordinates, bond distances and angles and anisotro-
pic displacement parameters in CIF format. CCDC 853818. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2gc16679h
Scheme 1 Reagents and conditions: (i) cyclopentadiene, BCl3; (ii)
LiAlH4, Et2O; (iii) 1,4-dibromobutane, K2CO3, MeCN reflux 15 h; (iv)
PhCH2Br, acetone; (v) cis-cyclooctene,
2 mol% [RuCl2(PCy3)2-
(vCHPh)], CHCl3, 40 °C, 15 h; (vi) ethyl vinyl ether, [P(CH2OH)4]Cl,
KOH, excess KBr, CHCl3, 50 °C; (vii) H3[PO4{WO(O2)2}4](aq), pyri-
dine, EtOH.
This journal is © The Royal Society of Chemistry 2012
Green Chem., 2012, 14, 925–929 | 925

View Article Online
Fig. 1 Molecular structure of 1-benzyl-1-((2-methylbicyclo[2.2.1]hept-
5-en-2-yl)methyl)pyrrolidin-1-ium bromide (4), confirming the stereo-
chemistry of cycloaddition. Hydrogen atoms, the water molecule of crys-
tallisation and the bromide anion have been omitted for clarity.
Fig. 3 TEM images of 5 (a) before catalysis and (b) recovered catalyst
after use in the hydrogen peroxide-mediated epoxidation of cis-cyclo-
octene showing some aggregation.
narrow monomodal molecular weight distribution (Fig. 2). The
ratio of pyrrolidinium bromide to cyclooctene incorporated into
the polymer was determined to be 0.5 from elemental analysis,
which corresponds to an x value of 50/3, based on the average
molecular weight of 5. Analysis of the polymer by ICP-OES
revealed the ruthenium content to be less than 0.001 wt%. The
thermal stability of the copolymer 5 was investigated by thermo-
gravimetric analysis and differential scanning calorimetry. The
TGA showed two main degradation stages (ESI†) and indicated
that copolymer 5 was thermally stable up to 170 °C, which is
above the reaction conditions required for the liquid phase
catalysis.
Fig. 2 Differential refractive index (dRI) GPC trace of polymer 4 in
DMF (0.6 mL min−1).
the absence of solvent and occurred with high exo-diastereo-
selectivity to give 1 as a 89 : 11 mixture of exo and endo diaster-
eoisomers. LiAlH4 reduction of 1 gave the corresponding
methylamine adduct 2, a versatile intermediate for the synthesis
of a range of onium-based monomers. The pyrrolidine ring of 3
was formed by intramolecular dialkylation of 2 with 1,4-dibro-
mobutane and subsequently quaternised by reaction with benzyl
bromide in acetone to afford the desired pyrrolidinium-based
monomer 4, which was isolated as a spectroscopically and ana-
lytically pure white solid. Importantly, although monomer 4
requires a linear four step synthesis, it can be accomplished
without purification by column chromatography, which greatly
simplifies the process and reduces the overall cost.
The Lewis acid-catalysed Diels–Alder cycloaddition between
methacrylonitrile and cyclopentadiene has recently been reported
to occur with high endo-diastereoselectivity,10 therefore a single
crystal X-ray structure determination of 4 was undertaken to
unequivocally establish the stereochemistry. A perspective view
of the molecular structure of one of the crystallographically inde-
pendent molecules is shown in Fig. 1, confirming that the major
adduct is in fact the exo-diastereoisomer, consistent with an
earlier assignment.11
ROMP of 4 with cis-cyclooctene was catalysed by 2 mol%
[RuCl2(PCy3)2(vCHPh)] in chloroform at room temperature.
Following polymerisation, the reaction was quenched with ethyl
vinyl ether and the catalyst removed by extraction with an
aqueous solution of tris(hydroxymethyl)phosphine, according to
a protocol developed by Pederson et al.12 The molecular weight
of 5 determined by gel permeation chromatography (GPC) was
measured to be 9100 Da (Mw), relative to polystyrene standards,
in good agreement with the theoretical value based on complete
consumption of monomer as confirmed by 1H NMR spec-
troscopy. The polydispersity of 1.03 is also consistent with a
The peroxophosphotungstate [PO4{WO(O2)2}4]3− was
selected for immobilisation because it is an efficient catalyst for
alkene epoxidation and alcohol oxidation and as such was used
to assess the relative merits of PIILP catalysis. Catalyst 6 was
prepared by stoichiometric exchange of the bromide anions of 5
with [PO4{WO(O2)2}4]3−, generated by hydrogen peroxide-
mediated decomposition of the heteropolyacid H3PW12O40;13
the product precipitated as an insoluble white amorphous solid
immediately upon mixing. Decomposition of heteropoly acid
H3PW12O40 into [PO4{WO(O2)2}4]3− was confirmed by a signal
at δ 2.8 ppm in the solid state 31P NMR spectrum, which also
showed the presence of several unidentified phosphorus-contain-
ing species previously observed by Hill and co-workers during
their study on the formation, reactivity and stability of
[PO4{WO(O2)2}4]3− (ESI†).13f Nitrogen sorption analysis of 6
gave a BET surface area of 42 m2 g−1 and pore size distribution
curves calculated by Barret–Joyner–Halenda method gave an
average pore size of 8.3 nm and a pore volume of 0.15 cm3 g−1
.
These values are consistent with similar polyoxometalate-based
polyammonium cation hybrid mesoporous materials with surface
areas of 27–51 m2 g−1, albeit with a smaller pore size of
3.6 nm.6 Analysis of 6 by transmission electron microscopy
(TEM) showed that the polymer is granular in nature with small
highly contrasting features of ca. <1 nm, corresponding to the
peroxophosphotungstate, surrounded by the poorly contrasting
cation-decorated co-polymer (Fig. 3a). ICP-OES analysis of 6
gave a tungsten content of 26.0 wt%, which is consistent with
the proposed formulation.
The efficiency of 6 was investigated for the epoxidation of
allylic alcohols and alkenes as these reactions have been cata-
lysed by peroxophosphotungstate-based systems immobilised on
926 | Green Chem., 2012, 14, 925–929
This journal is © The Royal Society of Chemistry 2012

View Article Online
Table 1 Epoxidation of allylic alcohols and alkenes with hydrogen
peroxide in acetonitrile catalysed by 6 and 7a
Entry
Substrate
Catalyst
Time
Conversionb,c
1
2
3
4
5
6
6
7
6
7
6
7
4
4
4
4
4
4
>99 (94)
74 (66)
89 (81)
87 (74)
>99 (92)
94 (86)
7
8
9
10
11
12
6
7
6
7
6
7
4
4
12
12
4
96 (91)
97 (89)
88 (81)
82 (71)
98 (91)
74 (68)
4
13
14
6
7
4
4
89 (79)
80 (71)
Fig. 4 Conversions with respect to time for the hydrogen peroxide-
mediated epoxidation of cis-cyclooctene catalysed by 6.
15
16
6
7
4
4
81 (72)
98 (89)
17
18
6
7
5
5
76d
and no make-up quantity of fresh catalyst was added. The reusa-
bility data for 6 showed that the epoxidation of cyclooctene went
to completion each time and the catalyst 6 recycled effectively
with only a minor reduction (∼6%) in reaction rate after each
recycle (Fig. 4). Analysis of the solvent after filtration and recov-
ery of the catalyst revealed that the tungsten content was too low
to be detected by ICP-OES (i.e. <1 ppm), confirming that leach-
ing was negligible. The reduction in conversion observed on suc-
cessive recycles is thought to be due to attrition during filtration
and recovery of the catalyst; this will be further investigated by
performing scale-up recycle experiments to improve and quan-
tify catalyst recovery and modifying the surface properties and
ionic microenvironment to improve catalyst–surface interactions,
substrate accessibility and efficiency. Interestingly, the TEM
image of 6 recovered after the 4th recycle in the catalytic epoxi-
dation of cis-cyclooctene (Fig. 3b) shows some aggregation of
the peroxophosphotungstate; however, the solid state 31P NMR
spectrum of recycled catalyst 6 showed that the tetranuclear per-
oxophosphotungstate [PO4{WO(O2)2}4]3− is stable, in agre-
ement with earlier studies by Ishii et al. and Venturello et al.13
79d
a Reaction conditions: 0.5 mol% 6 or 7, 1 mmol substrate, 2 mmol 35%
H2O2, 3 mL MeCN, 50 °C. b Determined by GC analysis of the reaction
mixture using decane as internal standard. Average of three runs.
c Isolated yield in parenthesis. d Determined by 1H NMR spectroscopy.
the surface of silica,4 polymers and polycationic dendrimers,5,8
as well as in ionic liquids.14 Parallel catalyst testing was also
undertaken with [PO4{WO(O2)2}4][NEt4]3 (7); full details are
given in Table 1. Our initial evaluation focused on the epoxida-
tion of trans-hex-2-en-1-ol by hydrogen peroxide using 0.5 mol
% catalyst in acetonitrile at 50 °C and, under these conditions, 6
gave 99% conversion after 4 h compared with 74% for the 7,
over the same reaction time (entries 1 and 2). Table 1 shows that
6 also gave good to excellent conversions for the epoxidation of
a range of allylic alcohols and alkenes and, encouragingly, in the
majority of cases outperformed 7. Epoxidation of 3,7-dimethyl-
2,6-octadien-1-ol occurred with complete regioselectivity for the
allylic alcohol to give the 2,3-epoxy alcohol as the sole product
in good yield (entries 7 and 8). Although 6 was less effective for
the epoxidation of homoallylic alcohols (entries 9 and 10), good
conversions could be obtained over longer reactions times. Grati-
fyingly, the conversions obtained with 6, for the epoxidation of
cyclooctene, approach those recently achieved with 9- and 27-
armed peroxometalate-cored dendrimers5 as well as peroxometa-
late-based room temperature ionic liquids.14b Future work will
systematically explore the efficiency of this PIILP catalyst as a
function of polymer microstructure, ionic environment and
porosity with the aim of optimising performance and broadening
substrate scope.
Conclusion
In conclusion, ROMP has been used to prepare a novel linear
cation-decorated polymer based on a pyrrolidinium monomer
and cyclooctene co-monomer. The derived peroxophosphotung-
state polymer-immobilised ionic liquid phase catalyst is an
efficient system for the epoxidation of allylic alcohols and
alkenes and, encouragingly, the catalyst could be recovered in an
operationally straightforward procedure and reused with only a
minor reduction in performance. Future studies will aim to
exploit ROMP as a tool for the controlled assembly of new
highly tuneable support materials and to this end we are cur-
rently (i) exploring what factors influence catalyst performance
with the aim of identifying an optimum catalyst–support combi-
nation for use in a continuous flow process, (ii) elucidating pro-
perty–function relationships, (iii) establishing whether the
polymer chain of the catalyst support has been epoxidised, (iv)
investigating whether functional co-monomers can be incorpor-
ated into the polymer chain to modify chemo- and enantioselec-
tivity, (v) developing novel architectures such as nanocapsules
Reasoning that a polymer-immobilised ionic liquid phase
support should effectively retain the immobilised peroxophos-
photungstate, catalyst reusability experiments were undertaken in
order to assess the longevity of the catalyst and the potential for
incorporation into a continuous flow process. The catalyst reusa-
bility was studied up to four cycles, by recycling the catalyst
recovered after centrifugation and siphoning off the liquid phase
at the end of each cycle. Recovered catalyst was used as such
This journal is © The Royal Society of Chemistry 2012
Green Chem., 2012, 14, 925–929 | 927

View Article Online
and polymeric micelles and (vi) extending the concept of PIILP
catalysis to a wider range of transformations.
precipitation of an amorphous white solid. The mixture was
cooled to 0 °C, filtered through a sintered glass frit and the pre-
cipitate washed with water (2 × 10 mL) and diethyl ether (3 ×
75 mL) and dried under vacuum to afford 6 in 84% yield. FT-IR
(KBr plates): ν˜ = 1086, 1058, 1035 (P–O), 957 (WvO), 837
(O–O), 563, 535 W(O2)s,a; Anal. Calc for C108H168N3O24PW4:
N, 1.58; W, 27.66. Found: N, 1.46; W, 26.00.
Experimental
Ring opening metathesis polymerisation of 1-benzyl-1-
((2-methylbicyclo[2.2.1]hept-5-en-2-yl)methyl)pyrrolidin-1-
ium bromide with cis-cyclooctene
General procedure for catalytic epoxidations
A flame-dried three-neck round bottom flask under a nitrogen
atmosphere was charged with chloroform (80 mL), cis-cyclo-
octene (3.0 mL, 23.0 mmol) and 4 (4.23 g, 11.7 mmol). To this
was added a solution of [RuCl2(PCy3)2(vCHPh)] (0.571 g,
0.694 mmol) in chloroform (ca. 10 mL) and the resulting
mixture was heated at 40 °C and left to stir for 19 h. Upon com-
pletion the reaction was allowed to cool to room temperature,
ethyl vinyl ether (0.69 mL, 7.0 mmol) added and the solution
stirred for an additional hour. The polymer was precipitated by
slowly adding the reaction mixture portion-wise to diethyl ether
(ca. 600 mL) with vigorous stirring; after stirring for a further
60 min the polymer was isolated by filtration, using a sintered
glass frit, washed with diethyl ether and dried to yield 5.2 g of a
pale green solid. A solution of tris(hydroxymethyl)phosphine
was prepared by degassing 2-propanol (90 mL) with nitrogen for
30 min prior to adding tetrakis(hydroxymethyl)phosphonium
chloride (2.6 mL, 18 mmol). Potassium hydroxide (1.0 g,
18.0 mmol) was added slowly over 15 min to the vigorously
stirred solution during which time a white precipitate formed.
The mixture was allowed to stir for an additional 10 min and
then added to a solution of the polymer in chloroform (ca.
100–150 mL). After heating at 60 °C for 19 h, NaBr (18.52 g,
180 mmol) was added and the mixture stirred for an additional
3 h at 60 °C. The mixture was then filtered, washed rigorously
with distilled water (3 × 50 mL) and the resultant organic layer
added dropwise to diethyl ether (ca. 500 mL) with vigorous stir-
ring. After stirring for a minimum of 60 min the polymer was
allowed to settle, isolated by filtration through a frit, washed
with diethyl ether (2 × 50 mL) and dried under high vacuum to
A Schlenk flask was charged with substrate (1.0 mmol), catalyst
(2 mol% based on W) and acetonitrile (3 mL) and the resulting
mixture heated at 50 °C with rapid stirring. The reaction was
initiated by the addition of hydrogen peroxide (35% solution,
0.18 mL, 2 mmol) and stirred for the allocated time. After the
reaction mixture had cooled to room temperature decane
(0.195 mL, 1.0 mmol) was added and the resulting mixture was
then diluted with diethyl ether (25 mL) and washed with water.
The organic layer was separated, dried over magnesium sulphate,
concentrated under reduced pressure and analysed by GC-MS to
determine the conversion before being purified by column
chromatography.
Acknowledgements
We gratefully acknowledge Newcastle University for financial
support (JRE), Mr Alex Jackson for his assistance with GPC
analysis, and the EPSRC grant CASTech. Solid-state 31P NMR
spectra were obtained at the EPSRC UK National Solid-state
NMR Service at Durham and high resolution mass spectra were
obtained at the EPSRC National Mass Spectrometry Service
Centre in Swansea.
Notes and references
1 (a) M. T. Pope and A. Muller, Angew. Chem., Int. Ed. Engl., 1991, 30,
34; M. T. Pope and A. Muller, Angew. Chem., 1991, 103, 56; (b) J.
T. Rhule, C. L. Hill, D. A. Rud and R. F. Schinazi, Chem. Rev., 1998, 98,
327; (c) M. T. Pope and A. Muller, Polyoxometalate Chemistry: From
Topology via Self Assembly to Applications, Kluwer, Academic, Dor-
drecht, 2001.
1
afford 5 as a buff brown solid in 61% yield (4.1 g). H NMR
(399.78 MHz, CDCl3, δ): 7.52–7.78 (br, C6H5), 7.24–7.50 (br,
C6H5), 5.14–5.52 (br,vCH), 3.41–4.11 (br, NCH2Ph
+
2 For a comprehensive and informative overview of the breadth of research
encompassed by POMs, see: Chem. Rev., 1998, 98, 1–390.
3 (a) I. V. Kozhevnikov, Catalysis by Polyoxometalates, Wiley, Chichester,
2002; (b) I. V. Kozhevnikov, Chem. Rev., 1998, 98, 171; (c) N. Mizuno,
K. Kamata and K. Yamaguchi, Top. Catal., 2010, 53, 876;
(d) N. Mizuno, K. Kamata, S. Uchida and K. Yamaguchi, in Modern Het-
erogeneous Oxidation Catalysis- Design, Reactions and Characterisation,
ed. N. Mizuno, Wiley-VCH, Weinheim, p. 185.
4 K. Yamaguchi, C. Yoshida, S. Uchida and N. Mizuno, J. Am. Chem.
Soc., 2005, 127, 530.
5 L. Plault, A. Hauseler, S. Nlate, D. Astruc, J. Ruiz, S. Gatard and
R. Neumann, Angew. Chem., Int. Ed., 2004, 43, 2924.
NCH2CMe), 2.17 (br, cyclopentane CH), 1.69–2.10 (br, cyclo-
pentane CH2 and CH andvCHCH2), 1.05–1.41 (br, CH3 and
cyclopentane and alkyl CH2); 13C{1H} NMR (100.52 MHz,
CDCl3, δ): 133.4, 133.2, 130.6, 130.4, 129.9, 129.2, 128.9,
32.6, 29.8, 29.6, 29.2, 29.1, 27.2, 21.8; Anal. Calc for
C36H56NBr: C, 74.20, H, 9.69, N, 2.40. Found: C, 73.82, H,
10.01, N, 2.35 (corresponding to a pyrrolidinium monomer
content of 62.5 mol%, 1.72 mmol g−1); GPC (average over 3
runs): Mw = 9100, Mn = 8600, PDI = 1.06.
6 M. V. Vasylyer and R. Neumann, J. Am. Chem. Soc., 2004, 126, 884.
7 A. Haimov, H. Cohen and R. Neumann, J. Am. Chem. Soc., 2004, 126,
11762.
Synthesis of polymer supported peroxophosphotungstate 6
8 R. Neumann and H. Miller, J. Chem. Soc., Chem. Commun., 1995, 2277.
9 Y. M. A. Yamada, C. K. Jin and Y. Uozumi, Org. Lett., 2010, 12, 4540.
10 A. Borosy, G. Fates, U. Muller and F. Schroder, Tetrahedron, 2009, 65,
10495.
11 W. L. Dilling, R. D. Kroening and J. C. Little, J. Am. Chem. Soc., 1970,
92, 928.
A hydrogen peroxide solution (35% w/w, 9.7 mL, 100 mmol)
was added to phosphotungstic acid (1.73 g, 0.6 mmol) dissolved
in a minimum volume of water and stirred at room temperature
for 30 min. After this time, pyridine (0.145 mL, 1.8 mmol) was
added followed by a solution of 5 (0.877 g, 1.8 mmol) in the
minimum volume of ethanol, which resulted in the immediate
12 R. L. Pederson, I. M. Fellows, T. A. Ung, H. Ishihara and S. P. Hajela,
Adv. Synth. Catal., 2002, 344, 728.
928 | Green Chem., 2012, 14, 925–929
This journal is © The Royal Society of Chemistry 2012

View Article Online
13 (a) C. Venturello and R. D’Aloisio, J. Org. Chem., 1988, 53, 1553;
(b) C. Venturello and R. D’Aloisio, J. Mol. Catal., 1985, 32, 107;
(c) Y. Ishii, Y. Yamawaki, T. Ura, H. Yamada, T. Yoshida and M. Ogawa,
J. Org. Chem., 1988, 53, 5387; (d) R. Neumann and A. M. Khenkin, J.
Org. Chem., 1994, 59, 7577; (e) A. L. Salles, C. Aubry, R. Thouvenot,
F. Robert, C. Doremieux-Morin, G. Chottard, H. Ledon, Y. Jeannin and
J.-M. Bregeault, Inorg. Chem., 1994, 33, 871; (f) D. C. Duncan, R.
C. Chambers, E. Hecht and C. L. Hill, J. Am. Chem. Soc., 1995, 117,
681.
14 (a) Y. Qiao, Z. Hou, H. Li, Y. Hu, B. Feng, X. Wang, L. Hua and
Q. Huang, Green Chem., 2009, 11, 1955; (b) H. Li, Z. Hou, Y. Qiao,
B. Feng, Y. Hu, X. Wang and X. Zhao, Catal. Commun., 2010, 11, 470.
This journal is © The Royal Society of Chemistry 2012
Green Chem., 2012, 14, 925–929 | 929